1. Field of the Invention
The present invention relates to an index table for a machine tool and, more particularly, to a construction of an index table for indexing the angular position of a workpiece, which index table is suitable for gear cutting and grinding.
2. Description of the Related Art
In a conventional gear grinding device, a gear to be machined, which is a workpiece, is set on a rotary index table, which is capable of being moved vertically, by using a collet chuck, the rotary index table is rotationally driven by a servomotor via a gear reducer means, and the rotational speed and the rotational angular position of the index table are detected by an encoder attached concentrically to the index table, the detected values being sent to a control unit and feedback controlled so as to match the preset values of the rotational speed and the rotational angular position of gear grinding.
A problem with the rotary index table having such a gear reducer means is that since power is transmitted via several gears, the table rotation has an angular position error due to backlash between the gears, which degrades the accuracy of the gear to be machined.
To solve this problem, the driving means for the conventional index table described in Japanese Unexamined Patent Application Publication No. 2003-159634 (FIG. 1) uses a planetary roller speed-reduction mechanism including a plurality of stages of backlash-less planetary rollers in place of the gear reducer means to obtain a predetermined speed reduction ratio.
Also, the index table of conventional example described in Japanese Unexamined Patent Application Publication No. 2003-25147 (FIG. 3) has a backlash-less configuration in which a driving motor is housed in a housing that supports the index table, and the output shaft of the motor is connected directly to the table on which the workpiece is set.
The conventional example described in Japanese Unexamined Patent Application Publication No. 2003-159634 has an advantage that a power transmitting means scarcely has backlash. In this conventional example, however, since a coupling is interposed between a motor and the planetary roller speed-reduction mechanism, many connecting parts such as the output shaft of motor, the coupling, and the planetary roller speed-reduction mechanism are present, so that a power transmitting part lengthens. Therefore, the rigidity decreases as a whole, and therefore an adverse influence may be exerted on the rotation accuracy.
Also, in the conventional example described in Japanese Unexamined Patent Application Publication No. 2003-25147, a power transmitting means has no backlash and has no problem with the rigidity as in the above-described case. However, since the motor connected directly to the table requires high rotation torque at a low speed, so that the size of device inevitably increases, and therefore the sizes of the housing and peripheral functional parts also increase, which presents a problem of high cost.